Water-injected screw compressors often are chosen above oil-injected or oil-free screw compressors.
With oil-injected screw compressors, the produced air in fact is not free of oil.
With classic oil-free screw compressors without an injection of lubricating liquid, the compressor part is connected to the motor by means of a gearwheel case with oil lubrication. Here, the rotational speed of the rotors is higher.
Compared to such classic oil-free screw compressor, a water-injected screw compressor has a number of advantages. First, an outlet pressure up to 15 bar can be obtained in one stage instead of in two or three stages. Therefore, the compressor can be realised less expensive and more compact. Also, due to the lower working temperature, coolers may be sufficient which are simpler than with the classic oil-free screw compressors. Moreover, the water-injected screw compressor has a lower noise production and a higher efficiency. Moreover, as no oil or grease is used, no additional maintenance is necessary and the compressor is more environmental-friendly as there is no waste oil or grease.
With the water-injected screw compressor, water is injected onto the rotors in order to cool, seal and lubricate the rotors, whereby the male rotor directly drives the female rotor.
It is obvious that in this application, by “water” not necessarily 100% pure water is intended. This water may comprise additives, such as anti-corrosive means and/or freezing point depressing means.
Water is also injected for lubricating the slide bearings in which the male and the female rotors are borne by means of axle ends.
In the compression chamber, the air is compressed and driven out together with the water through an outlet. The air-water mixture then is transported off to a vessel/water separator, where the major part of the water is separated. The water is collected in the vessel, at the bottom thereof, and the compressed air is transported off at the top.
The speed range of a water-injected screw compressor is higher than that of an oil-injected screw compressor, amongst others, due to the lower viscosity and the higher heat capacity of water. If a water-injected screw compressor should be driven directly by an electric motor, this motor thus would have to run faster than with an oil-injected screw compressor, which leads to problems for the bearing of the motor as well as for the cooling of the motor.